This invention relates to a melt blowing die apparatus for spinning filaments of molten synthetic fiber material to produce fibrous non-woven thermal insulating mats constructed of thermoplastic fibers and particularly, though not exclusively, to form high loft batts of linear condensation polymers, preferably polyester, for example, polyethylene terephthalate (PET). The mats may be thermally insulating mats in the form of mats, boards or batts with an insulating R value of at least 3.5/inch and preferably at least 4/inch. Specifically, the invention relates to control of the drawn filament by a flow of pressurized air flow parallel to the extruded filaments to provide attenuation of the filaments within an attenuation slot provided in a lower portion of the apparatus.
It has been proposed to produce polyester (e.g. PET) non-woven insulating mats, constructed by melt-blowing techniques, having R values of 4.0 or more per inch with mats using substantially continuous fibers of 3-12 microns. However, mass production of high-loft batts suitable for the insulation of building structures have not, in the past, proved economical in spite of extensive research efforts devoted to producing such environmentally friendly products.
PET non-woven fiber mats, specifically for insulating purposes, whether commercial or domestic, have been proposed using melt blowing die equipment in which melted PET is extruded through a plurality of nozzles to form substantially continuous fibers which are then carried by a high velocity gas toward a fiber mat forming location, at which the fibers are laid down with self entanglement, resulting from the highly turbulent accelerating gas flow, to produce the desired batt integrity. It is proposed in the art to produce such insulating batts (and boards) via one or more arrays of nozzles disposed in a straight line arranged over the mat forming location to progressively produce the desired batt configuration as it is conveyed under the array(s). As the fibers are extruded by the nozzles, they are collected on a collection device, in a layer of fibers to form the insulating mats, batts or boards.
U.S. Pat. No. 5,248,247 discloses an aligned nozzle configuration, two slot ducts producing air jets directed to intersect, at an acute angle, the spin line below the nozzle carrying die face (or near it). The role of the air jets is to cause the extruded polymer filaments to be stretched and expose the fibers to turbulent air flow and preferably broken up prior to deposition in a random mass on the moving belt below the die. The main thrust of the patent is directed at the provision of a uniform driving pressure along the entire lateral die length for the air supply system feeding the slot nozzles. It is postulated, in this prior art, that even small variations, along the die length, in this total driving pressure applied to the slot air flow will lead to an unacceptable non-woven product/mat.
Other components of the meltblowing die are elongate plates referred to as air knives (nozzle bars), which form an accelerating air flow channel to, in combination with the die tip nose piece, form converging air flows to attenuate and draw down the extruded fibers to microsized diameters. The air knives are generally elongate plates which have a longitudinal edge tapered to form a knife edge. Two air knives are typically used and are fastened to the face of the die body on opposite sides of the triangular die tip nose piece. The tapered edges of the air knives are aligned with the confronting tapered surfaces of the nose piece and spaced slightly therefrom to form two flow channels which converge at the apex of the nose piece.
The spatial relationship between the air knives and the die tip is defined in the art by parameters known as air-gap and set-back. The air-gap and set-back determine the geometry of the converging air flow passages, and thereby control the airflow properties and the degree of fiber-air interaction.
The prior art melt blowing apparatus as disclosed in U.S. Pat. No. 5,248,247 for production of melt blown filament line is shown generally in FIG. 1 as comprising an extruder 1, melt blowing die 4 and a collector drum or conveyor belt 12. The extruder 1 delivers molten resin through an aligned evenly spaced series of nozzles 6 in the die 4, where, upon exiting the nozzles 6, an aligned evenly spaced plurality of filaments (fibers) 2 are extruded to be attenuated and passed down through tapered slits, in a lower portion of the apparatus onto the conveyor belt 12, by pressurized, converging hot air streams. The tapered slit 11 is formed by adjacent parallel relatively thin nozzle bars 5 through which the combined air/fiber stream passes. The filaments 2 are then collected on the belt 12 to form a mat or fleece of insulation F.
The melt blowing apparatus also includes a source of pressurized air 3 communicating with the die 4 through valved lines 8 and heating elements 7 in order to produce the converging hot air streams 9. Additionally, baffles and air pressure regulating devices 10 are provided together with the heating elements 7 and valved lines 8 to control the conditioned hot air streams 9.
As is known to those in the art, the extruder 1 includes an interior cavity where PET chips or similar polymer material are pressurized, heated and melted to produce the molten PET resin. The extruder 1 is provided with the aligned evenly spaced plurality of nozzles 6 communicating with the cavity. The nozzles 6 are supplied with molten PET under pressure to form an aligned evenly spaced plurality of filaments 2 at a desired flow rate.
In conjunction with the molten resin flow, the hot air streams 9 are provided from the pressurized air source 3 via the valved lines 8 into confluence with the filament line 2 substantially adjacent the nozzle 6. The hot air streams 9 are directed by an outlet oriented so as to introduce each of the air streams into the slit 11 at an acute angle to the direction of the flow of the filaments 2, thereby attenuating and drawing the filaments 2 downwards towards the conveyor belt 12 as illustrated in FIG. 1.
The slit 11 does not provide parallel flow controlling walls and is formed by the relative thin nozzle bars in which the slot forming walls converge throughout the vertical height of the slot and thereby fail to provide a controlled flow of the air, passing therethrough, parallel to the filaments and thus do not provide adequate control of filament attenuation and temperature. Here no mention of controlling the temperature of the slot walls is made.
It is an object of the present invention to provide an improved melt blowing apparatus and method to more effectively control the properties of attenuated filaments formed thereby.
The main objective of the invention is to provide an attenuating air flow in a vertical direction, parallel to the exiting filament direction so that appropriate shearing forces may be applied to the extruded filaments in the attenuating channel. This objective is achieved by means of a small radius Coanda bend or by a suitably designed channel flow, immediately upstream from the entrance to the attenuating channel.
Another object of the invention is to provide adequate fiber entanglement below the die face and the exit plane of the slot discharge, by means of the highly turbulent flow field and the free air entrainment existing in this region.
Shearing forces applied to an extruding filament of molten synthetic fiber material (polymer) by a suitably configured air/gas flow system provide an important means of influencing and controlling the molecular orientation, crystallinity and crystal orientation in certain high speed fiber spin line applications. The control of both the magnitude and location of the applied shearing forces, through the design characteristics of the air/gas system, is crucial to the achievement of improved and optimized mechanical properties of the resulting drawn fibers.
In contrast to the above described prior art, the present invention provides an accelerating high velocity fiber attenuating air flow in a vertical, attenuating slot extending along the fiber length as it is extruded. At the entrance section of this slot, along the fiber center line, the extruded filaments move vertically downward with a relatively low velocity. In this slot surrounding these filaments is provided an accelerating parallel high velocity air flow, with a maximum velocity approximately two orders of magnitude greater than the emerging filament velocity. The air flow is supplied by two identical, mirror image, ducting systems symmetrically disposed one on either side of the die nozzle center line, with each incorporating a rapid turning section immediately upstream of the slot entrance, so that the air flow enters the attenuating channel flows in a substantially vertical (downward) direction. In the attenuating slot, shearing and attenuating forces and temperature quenching are applied to the extruded molten filaments. The final product""s physical properties are critically dependent on the magnitude and time/space histories of the shearing and temperature quenching applied.
At the exit of the attenuating slot, the air discharge from the attenuating slot emerges as a free turbulent jet quickly acquiring high turbulent energy levels. In particular, large lateral turbulent velocity components are developed due to free air entrainment. The latter contribute significantly to the entanglement of the now rapidly solidifying or solidified filaments collected below the slot exit plane.
An important element of the present invention relates to the supply of suitably conditioned attenuating air flow to the extruded (polymer) filaments. To develop the necessary shear forces at the air flow/polymer interface, the air flow must be delivered to the spun filaments (fibers) in a substantially vertical direction (i.e. parallel to the length of the filaments), at a point as close to the exit of the extruded filament from the nozzles as possible. The air flow must execute a very tight turn, approaching 90xc2x0, to arrive at the vertical direction at or near the top of the spin line, after traversing an approximately horizontal path across the die extruding components (die nozzles) by which the filaments are extruded. A Coanda bend in the air supply is a preferred means of achieving this separation free flow turning.
Two identical air flow channels symmetrically converge on the die center line at the top of the spin line, on either side of the extruded filaments. The converged air flows from the systems, together with the extruded filaments, enter an attenuating slot, where the main shearing forces and temperature quenching are applied to the molten filaments. The degree of temperature quenching is controlled by the temperature difference maintained between the extruded fluid filament, at the die exit, and the conditioned attenuating air supply used.
Two alternative attenuating air supply systems are described to meet the major design objectives/requirements of the present invention. These objectives are:
The mean air flow velocity must be increased significantly to a high subsonic value at the downstream delivery location at the top of the spin line. The outlet/inlet velocity ratio required in the air system is on the order of 10:1 to 20:1 with the exact value dependent upon the required filament shearing forces and the drawing/attenuation needed in the final product.
The air flow must be turned to a substantially vertical discharge direction, by means of a small radius of curvature turn, at or immediately above the flow discharge into the attenuating slot.
The rapid turning and acceleration of the mean air flow in the system must be achieved without the introduction of any adverse flow pressure gradients on the walls defining the flow passages in the air supply system.
The delivered high velocity air flow at the top of the spin line must be uniform, along the length of the die, and uniformly across the inlet to the attenuating slot.
In the first of the general design approaches, FIG. 2 reveals a suitably configured and curved, fully attached internal flow channel to deliver the necessary air to the spin line. The air flow channel has a general xe2x80x9cSxe2x80x9d shaped center line contour, with the first, low speed turn directing the air flow entering (approximately vertical) across the bottom of the high pressure polymer nozzle assembly towards the spin line. The second, high speed turning in the xe2x80x9cSxe2x80x9d channel orients the discharge flow into the vertical spin line direction with a small radius bend. The air flow acceleration in the channel is such that high accelerations are applied in the low-velocity sections of the channel, including the first, low speed, turn, while small and vanishing accelerations are applied in the high-velocity sections including the second, high speed, turn. The final high speed turn must be carried out using a relatively small radius bend in order to permit the application of air shearing forces vertically at or near the top of the spin line. The entering air in the supply system is at a low velocity determined by the supply ducting and the blower/fan/compressor used to produce the necessary supply of air pressure and volumetric capacity of the die system. The air supply system also includes a suitable air heating unit to provide appropriate control of the temperature in the drawing/attenuating processes in and below the attenuating slot section. The final air discharge velocity from the supply system will typically be in the Mach No. range between 0.50 and 0.75 (400-800 f.p.s.) although wider limits are not precluded.
In the second of the general design approaches, for the air supply system (FIG. 4) the second turn described above which turns the air flow to the vertical spin line direction, is replaced by a short, approximately horizontal, wall jet section and a two-dimensional Coanda bend of approximately 90xc2x0. The curved free surfaces of the wall jet and the Coanda bend are vented to atmospheric pressure, as shown, through a suitable ducting arrangement. These free Coanda surfaces located symmetrically on both sides of the spin line entrain a significant volume of vented air prior to the convergence of the wall jets at the top of the spin line, at the entrance to the final attenuating channel section. On either side of the spin line trapped and standing vortices may be maintained above the curved free jet surfaces. Recirculation into the flow volume containing the trapped vortices must be terminated by a suitable wall contour design, prior to the convergence of the two Coanda wall jets at the entrance to the attenuating channel section. Coanda wall turns provide excellent flow turning properties when properly designed and vented. With turning radius to jet thickness ratios in the region 4-6, total turning angles of greater than 130xc2x0 can be achieved without wall separation.
Acceleration rates of the air/gas flow in the discharge channel are set at levels appropriate to the desired axial strains to be applied to the attenuating fiber filaments. The necessary flow accelerations are provided through appropriate area and geometry variations incorporated into the discharge nozzle design.
Additional control of the drawn filament properties in the drawing scheme described, can be obtained by adjusting and controlling the temperature difference between the extruded polymer filament and the quenching air/gas flow utilized.
In certain applications, it may prove advantageous to provide the necessary gas/air flow turning into the spin line direction, turning this flow into the spin line direction, by combining a Coanda bend section with a suitably curved fully attached channel flow section. Thus the total required flow deflection would be achieved in separate, but connected, channel sections.
A Coanda jet is a term applied to a class of jet flows having the following features: i) a thin wall jet flow discharging over a straight or an arbitrarily curved wall surface, and in continuous contact with this surface, at one edge (side), so that entrainment at this edge is entirely eliminated; ii) the remaining (outer) jet edge is exposed to a constant pressure region when large free air entrainment occurs.
The feature of Coanda jet flows that is particularly attractive for present design purposes is the relatively very tight wall curvatures that can be negotiated without the expected separation of the jet flow from the wall surface. The wall jet may be either laminar or turbulent, however, for present applications a laminar flow is preferred.
The most important inventive aspect of this submission would appear to be as follows: i) provision for an abrupt turn and acceleration of the attenuating air flow into the spin line direction, without wall separation to accomplish the required turning flow and ii) the application of the major attenuating forces to the filaments internally in an attenuating slot. The magnitude and axial variation of the magnitude of the applied shear forces are controlled by the design of the channel section and the temperature of the supplied attenuating air flow. In particular the axial variation of the channel flow area is an important design consideration. For the formation of non-woven mats from PET the following parameters of the present invention are typical:
Extrusion die head temperature of 500/700xc2x0 F.
Filament Velocityxe2x80x94exiting the polymer nozzle of about 0.1 to about and exiting the die slot with a velocity in the range of about 20 to about 200 feet per second. Large variations in both of these are to be expected, with a factor of plus/minus, three/four quite probable (both depend on the die design objectives).
Air Flow Velocityxe2x80x94exiting the polymer nozzle≈400/800 f.p.s. Again large variations can be expected (design dependent) with an upper (sonic) limit of approximately 1200/1400 f.p.s.
Filament Diameter Attenuation ratio 10:1 to 100:1.
Original Typical Filament Diameter of about 0.01-0.02 inch.
Attenuating Slot Width/Height
widthxe2x80x940.10-0.50 inch
heightxe2x80x940.25-2.50 inches
Die clearance above Tablexe2x80x942 to 20 ft. typical.
Temperature of Attenuating Air (Die Entrance)xcx9c500/700xc2x0 F., typical.
Temperature of Entrained Air from ambientxcx9c+50xc2x0.
Diesxe2x80x94heatedxcx9c400/700xc2x0 F., typical.
Two general design approaches for the air supply system required are sketched in FIGS. 2 and 4. FIG. 2 configuration does not incorporate the Coanda effect of FIG. 4 to achieve the required flow turning. In FIG. 2, turning is accomplished via duct wall design, with the polymer exterior surface providing the inner duct wall profile. The air flow in the case, is smoothly and rapidly accelerated, through a large area contraction (10:1) by means of cubic wall profiles, and simultaneously turned into the spin line at the base of the polymer nozzle. A very accurately controlled wall profile is required throughout the length of the die, to avoid air flow separation in the resulting xe2x80x9cSxe2x80x9d shaped nozzle.
According to the invention there is provided a melt blowing die apparatus, for extruding a plurality of polymer filaments for the manufacture of non-woven thermally insulating polymer mats, comprising: a) a die having a downwardly facing die face, defining a plurality of polymer filament extruding nozzles having axes directed to extrude the filaments vertically downwardly; b) a slot defined by vertical opposed parallel side walls evenly spaced on opposite sides of the axes, through which the filaments, extruded by the die through the nozzles, pass; and c) a pair of air supply channels located adjacent the downwardly facing die face, one on either side of the axes, each for the supply of a hot air stream vertically downwardly to and through the slot on opposite sides of said axes in contact with the filaments to attenuate the filaments passing vertically downwardly through the slot thereby to produce attenuated filaments to form the mats subsequent to downward exit from the slot.
Also according to the invention there is provided a method of melt blowing polymer filaments, for the manufacture of non-woven thermally insulating polymer mats, comprising the steps of: a) extruding a plurality of polymer filaments downwardly; b) passing the filaments centrally through a slot, having vertical parallel slot defining side walls, common to all the filaments; c) providing heated air streams on opposite sides of the filaments, to flow vertically with the filaments through the slot to attenuate the filaments while in the slot and below to produce attenuated filaments for the formation of the mats.